The present invention relates generally to dimming applications for lighting systems. More particularly, the present invention relates to a non-isolated dimming interface for a lighting device such as an electronic ballast or LED driver, having inherent overvoltage protection and the ability to dynamically adapt for operations with either of an analog or digital dimming device.
One purpose of an invention as disclosed herein is to provide a dimming interface for a lighting device (such as, e.g., an LED driver or electronic ballast) that can deliver a trickle current for an analog dimming interface or a digital load with digital communication capability. It would be desirable in view of the lack of practical alternatives in the prior art to further provide a dimming interface which is a voltage limited constant current source capable of changing the level of the driven current depending on whether it is delivering current to an analog interface or whether it is delivering a much larger current to a load.
It is conventionally known in the art to provide circuitry for protecting the dimming control interface in lighting devices against line voltages. In response to the application of line voltages, high impedance is often provided to limit current in the protection circuit, and clamping circuitry may be further provided to limit the output voltage from the protection circuit to the interface circuitry and the remainder of the lighting device generally. However, such circuits typically also utilize PTC thermistors or high voltage transistors to provide such protection, which increases the cost of the circuit. Accordingly, it would be desirable to provide a relatively low cost interface circuit with sufficient protection against the application of line voltages.
One example of a dimming interface circuit and method may be described with reference to U.S. Pat. No. 6,144,639, wherein a DC voltage is applied through a resistor network and a diode connected to the positive terminal, and an FET connected to the negative terminal, to establish a constant current from the analog interface circuit. Under normal loading conditions, the FET connected to the negative terminal is biased on so as to allow the constant current to flow out of the DC voltage source's positive terminal. An analog control signal is sensed between the diode and the current limiting resistors. If a large voltage was to be applied to the output terminals, either the diode would block current being driven into the circuit via the positive output terminal, or the high voltage FET would be negatively biased so as to block current being driven into the circuit via the negative output terminal. The diode and FET would accordingly protect against misapplication of the mains input voltage.
However, one disadvantage to this method is the fact the diode will distort the measure of the analog control voltage. Also, this circuit has no provisions for changing to a high current output to power (and potentially communicate with) an external dimming device.
Another conventional method may be briefly described by reference to FIGS. 1 and 2. This alternate method includes an interface circuit 100 that develops a constant current and measures the analog control voltage via an isolation transformer T1. An FET Q1 applies a DC power source to a primary winding of the flyback transformer T1B, which supplies current to a secondary winding T1C, and further enables sensing of the output voltage across first and second dimming interface wires (illustrated as “violet” and “grey”) via a tertiary winding T1A.
In the circuit as shown, further depending on the component values and size of the associated components, misapplication of the mains input voltage can damage the secondary components. The resistors R3, R4, R5, and R8 in the grey wire return path of the secondary circuit in FIG. 2 develop an offset voltage that is transferred to the tertiary winding T1A. As such, the offset must be subtracted before attenuating to a useable level, which requires a relatively expensive, low supply current reference voltage device D7 to subtract off most of the offset.
To reduce construction costs for the isolation transformer and associated circuitry, much of the circuitry from FIGS. 1 and 2 can be moved to use the same circuit ground as the output LED drive circuitry. However, while this can reduce costs, components such as shunt regulator U1 become vulnerable to mis-wiring, which leads to field failures.